Increased free radicals in vivo are associated with various deseases such as cancer and atheroclerosis. No effective method, however, is avaliable to directly measue in vivo free radical susceptibility. In this proposed study, we plan to evaluate the suitability of a spin trapping method in directly detecting free radicals induced by cigeratte smoking in smokers. Twenty smokers and non smokers will take 1 gram of aspirin before urine collections on three separate occasions. 2,3-DHBA will be measured to determine the level of free radical production. The level of 2,3-DHBA will be compared with other biomarkers in urine and blood that have been used for indirect assays of oxidative stress. The stability and repeatability of the urine salicylate assay in humans will be also evaluated in the study.